I'm currently behind on reviews, so don't be surprised if the recent reviews are a bit sparse.

The Arithmancer

Cover of The Arithmancer.

Honestly decent Harry Potter fanfic: Hermione starts out a math genius and things escalate. Good writing, a bunch of very thoughtful world-building. Close to canon without being stupid or pretentious. Delightfully nerdy, good research. Overdoing the canon in places, I think, and also the meta fandom commentary (but not often and not obnoxiously).